"The current credit crunch is a head cold compared to the double pneumonia this country will suffer if we don't implement an energy policy urgently," she told reporters.

"That is why security of supply now takes priority over everything, even climate change.

"If we are going to cope with climate change, it is going to cost money; if we want to protect the environment, it is going to cost money; and if we want to change to a low-carbon economy, it is going to cost money."

The report identified a number of factors that would combine to create the energy gap.

It said the main impact would be the loss of 23 gigawatts (GW) of electricity generation capacity between now and 2020.

The UK's ageing nuclear reactors, which currently provide about a fifth of the nation's electricity, are set to be decommissioned over the coming years.

Current projections show that by 2023, the UK will have only one nuclear reactor in operation.

And an EU Directive that requires the most polluting coal- and oil-fired power station to close would result in the likely loss of a further 12GW generation capacity.

Mind the gap

In its 2007 Energy White Paper, the government indicated that it was in favour of a new fleet of nuclear power stations.

But co-author Ian Fells said this strategy failed to address the short-term shortfall.

"Let's put it this way, the current UK energy policy is not fit for purpose," he told reporters at a briefing in central London.

"Something has to be done about it if we are not going to run into serious problems in the middle of the next decade."

Professor Fells warned: "It is all very well for the prime minister to decide that we need to build a lot of new nuclear power stations, but implementing that is really rather difficult."

He said that it was likely to be a further decade before any new reactors came online, which would not be soon enough to help bridge the looming energy gap.

The report also questioned whether government subsidies for renewable energy were the best way to spend public money.

Professor Fells said: "We have reached the stage where there is tremendous emphasis on renewable energy, and rightly so because renewables need to be an important part of the energy mix.

"But it is totally failing to meet the targets that were set.

We will continue to take the tough decisions needed to ensure that we have reliable energy supplies in the decades ahead

Energy Secretary John Hutton

"The UK is supposed to generate 10% of its electricity [from renewable sources] by 2010, yet it will be about 6%.

"It is more worrying that we have signed up to the European energy plan, which is 20% renewable energy by 2020 - that implies about 40% renewable electricity.

He added that government figures showed that subsidies for renewables last year amounted to £1bn.

"If we continue the way we are providing subsidies at the moment, that would gross at between £20bn to £30bn by 2020.

"This is a staggering subsidy that is being provided to keep renewable energy on the road."

Out of touch

Mr Hutton said: "Ian Fells overstates the risk of the energy gap, but he also understates what the government's already doing to secure our future supplies and increase our energy independence - such as a tenfold increase in renewables, a renaissance of nuclear energy in the UK, and backing clean coal technology."

He added: "That's not to underestimate the task we've got on our hands. Securing future energy supplies for the UK is a matter of national security and so we're not going to rule out any radical options.

"That's why we keep our energy infrastructure under constant review, and will continue to take the tough decisions needed to ensure that we have reliable energy supplies in the decades ahead."

Greenpeace UK's chief scientist, Doug Parr, said the global appetite for renewables meant the UK had to position itself as a leading player.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.